PROJECT SUMMARY Studies in animal models have linked direct exposures to endocrine disrupting chemicals (EDCs) with the onset of disease in descendants of the exposed individuals. Many groups have demonstrated such transgenerational effects of chemical exposures, which are proposed to be examples of epigenetic inheritance. Although transgenerational effects have substantial support in the literature, the concept of inheritance in the absence of DNA sequence changes is controversial because the underlying mechanisms have not been satisfactorily explained. If we do not know how transgenerational inheritance of environmental exposures is transmitted, how can we incorporate the effects of these chemicals on disease burden into risk assessment paradigms that adequately protect public health? How can we determine which chemicals may have transgenerational effects? We have developed a transgenerational model for obesity. When pregnant F0 female mice are treated with environmentally-relevant (nM) doses of TBT via their drinking water, increased fat accumulation can be detected in at least the next four generations of descendents (the F1-F4 generations), even on a low-fat diet. Male F4 descendents of pregnant F0 dams treated with TBT throughout gestation developed a transgenerational ?thrifty phenotype?: they were resistant to fat loss during fasting, rapidly gained weight when dietary fat was increased modestly and retained this fat despite being returned to a normal, low- fat diet. Our published and preliminary results led us to propose a new model for transgenerational inheritance - that prenatal TBT exposure altered chromatin structure and accessibility, leading to regional changes in blocks of methylated DNA and differential expression of important metabolic genes, including the satiety hormone, leptin. We propose a comprehensive series of experiments designed to test the hypothesis that TBT induces transgenerational obesity by changing chromatin structure which is transmitted via the germ cells to subsequent generations. We propose the following Specific Aims to test this novel hypothesis: Aim 1: Identify what changes in genomic structure are elicited by TBT exposure in germ cells and how these are transmitted down the generations. Aim 2: What is the role of gonadal somatic cells in the transgenerational phenotype. Aim 3: What changes does ancestral TBT exposure elicit in the metabolome and can these be used to determine why the transgenerational obesity phenotype appears to be male-specific? Delineating these molecular mechanisms will greatly our knowledge of gene-environment interactions, should lay the groundwork for risk assessment that includes the contributions of transgenerational effects and will provide insights into how obesity can be prevented and the obesity epidemic curtailed - an important and timely public health issue.